The present invention relates to an electronic circuit and an array of such circuits for precisely measuring small amounts or small changes in the amount of charge, voltage, or electrical currents over a wide dynamic range.
Often the measurement of a physical phenomena involves the creation of an electrical signal which is amplified and measured by an electrical circuit. A number of sensing media face limitations, imposed by the electronic measuring circuits, on their ability to measure physical phenomena. These media include solids, liquids, or gases in which the physical phenomena to be sensed causes the generation of mobile charges which then move under the influence of an electric field, or in which a charge separation is induced by the physical phenomena and/or dimensional changes in the sensing medium. Specific examples of sensing media are gases in the case of ionizing radiation, electrochemical mixtures, electret or capacitive microphones, variable capacitors, inductive pickups or coils, electric field measuring antennas, piezoelectric materials such as PVDF, semiconductors operated at temperatures where thermally-induced current generation is not dominant, a vacuum in the case of electron emissive surfaces or ion mobility instruments, and insulators. The limitations imposed by the measuring circuits include sensitivity, linearity, size, dynamic range, and operating voltages. These limitations arise from thermally-induced and/or bias currents in the components of the measuring circuit and current leakage paths over the surfaces of and/or through the components of the measuring circuit. The measuring circuit described herein aims to minimize these limitations.
One example of a physical phenomena is ionizing radiation. It presents a direct hazard to people; therefore, the measurement of radiation in various environmental settings is important. The type of radiation monitor or detector used to measure the radiation depends upon the type of radiation, e.g., beta, alpha, or X-ray and the environmental setting, e.g., an environmentally isolated laboratory, an open mine, or a waste dump holding potentially hazardous material. Different environments impose different requirements on the manner and sensitivity of the measurement; for example, the laboratory most likely requires a monitoring system with a continuous display and singular or multiple radiation detectors; the mine requires a moderately sensitive portable area detector; and the waste dump a relatively fast and sensitive directional detector.
D. A. Waechter et al. described in an article entitled xe2x80x9cNew Generation Low Power Radiation Survey Instruments,xe2x80x9d a standard portable dosimeter (radiation monitor) system. The portable monitor consists of a Geiger-Muller tube (GM tube) with an event counter which records the number of ionizing events. There is a readout display and an audio alarm. The problem with the GM tube is that its response is not linear with the energy of the radiation so its accuracy varies with radiation photon energy, although it is useful for warning. In this instrument, the need to amplify the radiation signal in the GM tube limits the energy linearity and thus the accuracy of the instrument. An ion chamber made from tissue equivalent plastic and filled with a tissue equivalent gas gives a very accurate reading. However, at low doses and dose rates, the amount of charge generated per unit volume of gas is very small. For accurate measurement of the ionizing radiation, the signal current created by the ionizing radiation needs to be significant when compared to the leakage and/or noise currents in the electronic measuring circuit and the internal leakage currents in the gas sensing medium. It is preferred that the signal current be greater than the sum of all the leakage and noise currents. In general, the internal leakage currents of the sensing media, which are induced by physical phenomena other than the one to be measured, must be on the same order of magnitude or preferably less than the currents created by the physical phenomena to be measured in order for any measuring circuit, including the ones described herein, to obtain a measurement. Sensing media with low internal leakage currents are said to have high internal impedances. Gas is a very high impedance sensing medium and so does not contribute significant internal leakage currents. But, due to surface leakage currents on insulators and other circuit elements and due to other limitations within the implementing circuitry, prior art ion chambers tend to need a large volume of gas, and thus operate at high pressure or be inconveniently large, and need to employ high voltages to be sufficiently sensitive. By significantly reducing surface leakage currents and bias currents, this invention allows accurate ionizing radiation measurement with lower voltages and smaller chambers.
A second group of sensors that can benefit from the invention disclosed herein are charge inducing sensors such as capacitive sensors, where a charge is induced if the voltage difference is kept constant, or where a voltage is induced if the charge stays constant. Capacitive sensors can be used in many applications such as, microphones, pressure measurements, and accelerometers to name a few. Books like Capacitive Sensors Design and Application by Larry K. Baxter (ISBN 0-7803-1130-2) give many applications, some examples of which are given on pages 3-5. It is important to note that in capacitive sensors there is a high impedance material, often gas, but sometimes a liquid or a solid, between the capacitor electrodes. In this type of sensor the high impedance material is functioning as a separating medium rather than a sensing medium. The sensing medium is the capacitor plates themselves and the separation between them. There is not meant to be charge conduction through the separating medium. For this reason the term non-mobile charges can be used. The induced charges do move within the conductors connected to the sensing electrodes and the other electrodes, but they do not move through the separating medium. Most prior art capacitive sensors are limited to alternating current (AC) use because of the bias and leakage currents introduced by the prior art measurement electronics or within or over the surfaces of the capacitive sensors themselves. Among the limitations are: (i) response time, since it takes several cycles of the AC signal for a change to be noticed, (ii) power use, because the AC signal require an AC source and constant current draw, and (iii) sensitivity, because the measurement is a small change of a large AC signal.
Another group of charge sensors which can benefit from the invention is electrochemical sensors, where the phenomena to be measured interacts via a reversible or irreversible chemical reaction that causes the formation of charge in electrical dipole layers. The charges need not be mobile because the creation of an electric field by the induced charge distribution will result in a signal which can be measured. In some configurations, electrochemical sensors are low impedance sensors, with significant internal leakage currents inherent in the medium itself. Some water-based electrochemical cells are examples of this. Even in low internal impedance sensors, there may be a sensitivity or linearity benefit from using an electronic circuit which does not load or draw much current from the sensor. In some configurations, the current generated is the electrical signal which most directly relates to the physical phenomena. In other configurations, electrochemical sensors produce a voltage that is measured to represent the physical phenomena. The measurement of work function difference between two surfaces with an insulating medium as a separation material is an example of this. One prior art method for measuring work function difference is a moving plate or vibrating reed electrometer as described in pages 82-83 and pages 407-409 of Nuclear Radiation Detection by William J. Price (Second Edition, Library of Congress Catalog Card Number 63-23463). In this device, mechanically moving or vibrating a capacitor plate induces an alternating current proportional to the voltage difference between the two plates and the time rate of change of the capacitance, I=V*(dC/dt). While this circuit is very sensitive, its usefulness is limited because of the size of the vibrating plates and the power needed to create the vibration or motion. A vibrating reed electrometer is different than the capacitive sensors because in the capacitive sensors, the change in capacitance is related to the phenomena to be measured, while in a vibrating reed electrometer, the capacitance change is periodic and is used to amplify the voltage difference and convert it into a current.
Another set of applications in which the invention may be used are mass spectrometers, flame ionization detectors, vacuum gauges, gas chromatographs, photo-detector tubes, and others where charged ions or electrons are moving in a vacuum. These can be termed free electron detectors, although ions of both positive and negative charge may be collected, as well as free electrons. Often high voltage electron multiplication is used to amplify the signal. However, this amplification introduces some statistical noise and the electronics necessary to accomplish the high voltage multiplication are costly. The electrical circuit disclosed in the present invention can reduce noise and costs in these and similar charge measurement applications.
In all measurement or sensing applications where the noise of the electronic measurement circuit limits the lower limit of sensitivity or affects the linearity or dynamic range of the sensor, the electronic circuit presented herein can improve the sensor""s performance. Among types of noise are shot noise, Johnson and other thermal noise sources, bias currents, leakage currents and other charge conduction paths that interfere with accurately measuring or detecting the signal. It is also advantageous to use the circuit proposed here because of reduced cost, size, and/or power.
In U.S. Pat. Nos. 4,769,547, 4,804,847 and 4,970,391, certain circuits are described in relation to ion chamber construction. This patent application extends the teachings of those patents by disclosing circuit configurations and techniques which permit the invention""s use in other applications which can benefit from increased detector sensitivity, linearity, dynamic range, lower cost, etc.
One of the uses of this increased sensitivity is to increase the dynamic range of a sensor. But it then becomes important that the dynamic range not be limited by the electronic circuit which amplifies the signal. It is also usually desirable to have the measurement of the physical phenomena result in a digital number which can be subsequently used by a computer for analysis or control. This is usually done by an analog to digital converter (ADC), although some phenomena or electronic circuits can produce output that is digital without utilizing a separate ADC or a digital to analog converter (DAC) utilized as an ADC. The rate at which the digital number needs to change depends upon the bandwidth of the signal to be measured. The sample rate needs to be at least twice the highest frequency of interest to prevent aliasing.
Measuring ionizing radiation is an application where a large dynamic range is useful. The nominal background radiation is 100 mR/year. A chest X-ray has a dose rate of greater than 1R/sec. This is a dynamic range of current of more than 3xc3x97108:1. There would be great value in an instrument or a personal dosimeter that could measure over this whole range
Both the human eye and the ear have dynamic ranges on the order of 107. They use a number of techniques to achieve this wide dynamic range. Audio signal sensing is an especially demanding application because it requires the wide dynamic range at a relatively high frequency, up to 20 kHz.
For this disclosure, dynamic range refers to the ratio of the largest signal measurable to the smallest signal measurable. Precision refers to the smallest difference or change that can be distinguished. Precision has an inverse relationship to the size of the minimum signal that can be measured. When a person speaks about a circuit having greater precision, it means that a smaller difference between two signals can be distinguished. Sensitivity is also used to refer to the minimum signal that can be detected. Accuracy refers to the difference between the measurement of the input of interest and the actual value of that same input.
Among the widest dynamic range analog to digital converter (ADC) is the 24 bit sigma delta converter made by Analog Devices, the AD7710, protected by U.S. Pat. No. 5,134,401. This ADC gives a dynamic range of over 16 million to 1. But as can be realized from the FIG. DR4 which is a portion of the AD7710 data sheet, as the frequency of the signal being measured increases, the effective number of bits falls quickly.
There are many other types of ADCs, such as a flash ADC, a sub-ranging ADC, a pipeline ADC, a parallel time-interleaved ADC, a folding ADC, an interpolating ADC, a two-step ADC, a successive approximation ADC, an integrating ADC. The widest dynamic range of these is around 16 bits, a dynamic range of 65,000:1. Analog Devices has16 bit ADCs that can operate at 100,000 samples per second. Flash ADC""s are even faster, but because they use 2n comparators for an n bit converter, they are usually less than 8 bits, and most often 4 or 6 bits.
One common method to get around this dynamic range limit of speedy ADCs is to precede the ADC by a variable or programmable gain amplifier (PGA). If the gain is greater than 1, then smaller signals can be measured, but the maximum signal that can be measured is also decreased by the gain. If the gain is less than 1, then larger signals will not saturate the circuit, but the size of the minimum signal has been increased. In either case, having a programmable gain does not increase the dynamic range of the circuit, it just allows the dynamic range to be shifted. This is shown in Plot 2.
In the case of gas detectors or electrochemical cells where there is a small signal xe2x80x9criding onxe2x80x9d a significant offset, programmable gain stages will not work because no matter what the gain, the signal is of the same magnitude or significantly smaller than the offset.
By digitally feeding back a signal to various point along the amplifier signal path, the high precision can be maintained over a wide dynamic range.
In addition, the improved individual circuits disclosed herein can advantageously be combined into an array which can digitally accumulate information about a spatially dispersed or differentiated physical phenomena. The physical phenomena being sensed result in charge movement which is integrated by a digital counter at each array element for interfacing and communications with digital circuitry. The most familiar application is light sensing or imaging arrays. But arrays of other sensors are also extremely useful. Sound or pressure are two examples. An array can also be advantageous for phenomena such as chemical sensing, xe2x80x9ctastexe2x80x9d or xe2x80x9csmellxe2x80x9d where many sensors with slightly different properties or sensitivities are needed. Construction as an array simplifies manufacture, assembly and communications interfacing of the many circuits used.
The Charge Coupled Device (CCD) is a very common integrated circuit sensor array used to acquire an image. When light strikes the silicon crystal containing the CCD, free charges are generated and collected on capacitors in each of the 2 dimensional array of pixels. The amount of charge collected at a pixel is proportional to the amount of light that struck that pixel since the last time that pixel had been read or cleared. To read a pixel, electrode voltages are varied over time to move the collected charge along a predetermined shift path to the readout circuitry. Usually one row is read out at a time. When done in a controlled fashion, the readout signal as a function of time is representative of the light striking the array, usually presented in a raster scan of some type.
CCDs have several phenomena that limit their performance, including transport noise, pixel non-uniformity, readout noise, non uniformity, non-linearity, blooming and saturation, and dark current. These will be described in more detail, along with some of the prior art strategies for dealing with them. Transport Noise As charge is moved from cell to cell, the efficiency of transport is not 100%. This causes some smearing of charge from cells with more charge into cells behind them in the transport chain that have less charge. Also, some charge may be lost to recombination as it is transported. There are many manufacturing strategies and techniques to minimize this. The best way to eliminate it is to not move the charge packets at all, but measure their size, and convert that to a voltage or a digital number. This is taught in U.S. Pat. Nos. 5,461,425 and 5,801,657. These present a method to avoid transport noise by replacing the CCD with a phototransistor at each pixel, and then using an Analog to Digital Converter (ADC) at each pixel to convert the signal (the conductivity of the phototransistor) to a digital bit number. This digital number can be transported without further loss. However, phototransistors need to operate in relatively high light levels because they do not integrate the charge or the signal.
Pixel non-uniformity Pixels can differ slightly in size, charge creation efficiency, dark current, or the presence of charge recombination centers. This means that for a uniform input, the outputs from different pixels will not be identical. For a pixel A, the relationship between the output charge (C) over some time period t and the input signal intensity (I) can be expressed as C=XI+YI*Ixe2x88x92ZI*I*I. X is an offset, often expressed as the dark current. Y is a gain (charge creation efficiency), and Z is a loss due to recombination. For each pixel, the coefficients X, Y, and Z will differ. The best way to compensate for these differences is to convert the reading to digital numbers and use a digital processing to solve for I given the charge C measured at each pixel. A number of patents present this solution, with the difficulty being converting the analog signal to a digital number, storing the X, Y, and Z for each pixel, and then performing the computation quickly enough. An additional difficulty is that any blooming, saturation, smearing, or similar phenomena means that this equation no longer holds.
Readout noise, non-uniformity, non-linearity CCDs commonly use an amplifier per row. This also is a source of non-uniformity and non-linearity. While these come from a different physical phenomena, the effects are convolved with the individual pixel non-uniformities and non-linearities and so are dealt with using the compensation techniques described herein. Also, because each pixel is being read sequentially, only a small amount of time is available for the amplifier to settle before the pixel is read. This adds noise because the amplifier has to have a wide bandwidth to achieve the fast settling time.
Blooming and Saturation limit the maximum signal that can be imaged. Because charge is collected on a capacitor at each pixel, as charge is collected, the voltage across the capacitor increases. As the voltage increases, there is a tendency for newly arriving charges to move to adjacent pixels that are at a lower voltage. Also, as the charges are moving toward the pixels, their charge causes them to repel and spread out to some extent. This means that if there is one bright spot of light that strikes only one pixel, in the output image it will appear that the bright spot has xe2x80x9cbloomedxe2x80x9d to cover several pixels. Channel stops or charge drains can be added to eliminate blooming at the cost of increased complexity and reduced charge collection efficiency. But, even with these techniques to reduce or eliminate blooming, the signal still saturates. The signal is said to saturate when the charge collected no longer increases as the input intensity increases. The only way to hold more charge in a pixel is to increase the capacitance of the pixel by increasing its area or decreasing the insulator thickness, or by increasing the power supply voltages. The first and third are opposite the normal integrated circuit innovation trends of reducing feature size and reducing voltages. The only way to increase charge carrying capacity in the face of these trends is to use thinner insulators to create higher valued capacitors. However, larger capacitors are slower to switch, have reduced minimum light sensitivity, and thinner insulators are more likely to have pinholes, so even this reaches a limit of improvement.
It is an object of the present invention to provide an electronic circuit which improves the performance of sensors which produce small currents, charges, or voltages by improving their lower limits of detection, dynamic range, frequency range, and/or linearity.
It is another object of the present invention to provide an electronic circuit which improves sensor performance by eliminating continuous direct current discharge paths for removal of the charge or current generated by the sensor.
It is another object of the present invention to provide an electronic circuit which improves sensor performance by minimizing inaccuracies caused by bias currents, leakage currents, current drawn from or input to the sensor, and/or other charge conduction paths that interfere with accurately measuring or detecting the signal.
It is another object of the present invention to provide an electronic circuit which improves sensor performance by minimizing inaccuracies caused by shot noise, Johnson noise, thermal noise, and other AC or DC noise sources.
It is another object of the present invention to provide an electronic circuit which improves sensor performance by minimizing inaccuracies caused by the dead time during which the charge measurement circuit is being discharged or restored.
It is another object of the present invention to provide an electronic circuit for measuring current or charge.
It is another object of the present invention to measure a low current or low voltage signal and provide a direct digital output.
It is another object of the present invention to reduce the power required to measure a low current or low voltage signal.
It is another object of the present invention to provide an electronic circuit for use with a variety of sensing media, including but not limited to high impedance sensing media, that produce a signal by either charge or current, production or induction in response to some physical phenomena occurring within the sensing media.
It is another object of the present invention to provide small, lightweight assembly which electronically records the phenomena being sensed.
It is an additional object of the present invention to provide an electronic circuit with a transistor and/or integrated (electronic) circuit design, manufacture, and handling process that optimizes the ability to operate with small currents, small amounts of charge and high impedance sensing mediums.
It is an additional object of this invention to provide a wide dynamic range, DC coupled amplifier which can perform measurements over the whole dynamic range with a precision equal to that experienced at the minimum limit of the dynamic range.
It is a goal of this invention to provide a circuit which provides timely digital feedback for offset cancellation over a wide dynamic range.
It is an additional object of the present invention to provide an improved sensing array which minimizes or eliminates all the above sources of image noise by providing a direct digital accumulation and transmission of charge or signal information rather than performing an analog accumulation of charge per pixel and a subsequent digital readout and processing thereafter. A number of circuits to accomplish this are disclosed herein. These can be arrayed as needed. One circuit operates by collecting charge until the threshold of a comparator is reached, incrementing a digital counter when the comparator is triggered to register a measurement, and then either removing or compensating for the collected charge so that the signal from the comparator is returned to its untriggered state to allow it to register another measurement.
In many of these circuits the number in the digital counter represents the integral of the signal which has occurred since the counter had last been reset. Blooming is eliminated because charge never accumulates above a low threshold. Sensitivity can be increased because pixel capacitance can be reduced without increasing blooming. The saturation level is determined by the number of bits in the counter. To double the saturation limit, rather than doubling the area or capacitance of the pixel, another bit is simply added to the counter. This means that the trend to reduced feature size in integrated circuits will actually increase the saturation capacity of a pixel rather than decrease it because more counter bits can be placed in a given area. All transport of pixel image information is thereafter digital information, either serial or in parallel, with its inherent noise immunity. The digital output is also amenable to simplified improvement/compensation via various existing or yet to be developed correction and compensation strategies.
While the output from each pixel is a digital number, this circuit is an improvement on previous methods of having an analog to digital conversion per pixel of Fowler et al because it overcomes blooming and saturation to which prior circuits are susceptible. It also reduces the circuit noise because the conversion is taking place over the whole image acquisition period, rather than a much shorter time at the end.
It also is an improvement over circuits like those disclosed in U.S. Pat. Nos. 5,665,959, and 4,710,817 which use counters per pixel to count discrete events of highly ionizing radiation which generate a relatively large number of charges per event. Because xe2x80x9ceventsxe2x80x9d are counted, the pixel linearity and many of the blooming problems are avoided. However the number in the counters represent the number of events that exceeded a threshold, and does not represent the integral of the charge collected. These prior art methods are not adaptable to measuring phenomena where a continuous level signal is to be measured such as, light, sound, or electrochemical cell currents. Integration of the signal over time is also beneficial in improving noise immunity.
One embodiment of the present invention provides an electronic circuit for measuring current or charge that can be used with a variety of sensing media (including high impedance sensing media) that produce a signal by either charge or current production or induction in response to physical phenomena occurring within the sensing media. A current or charge sensing or collecting electrode is placed within the sensing medium to create a signal from the produced or induced current or charge that is electrically coupled to a control gate of an amplifying transistor which is either connected to or incorporated into an amplifier. The signal is created by establishing an electric field that moves charge of one polarity toward the sensing electrode. The electric field is established by setting a reference electrode at a different voltage potential (bias) than the sensing electrode. The control gate/sensing electrode, which is initially biased to a predetermined level, provides an output that changes with the amount of sensed charge to produce a signal at the amplifying transistor that is representative of the amount of charge collected. This signal is applied to an interface and passed to a sense amplifier so that when the signal passes a predetermined threshold, the counter is triggered and is incremented. Triggering the counter also commands a circuit element to clear or restore the predetermined bias level to the control gate (the sensing electrode) of the amplifying transistor. The control gate and interface are configured to have a very high impedance which creates some of the benefits of the invention. The counter circuitry provides a digital output representative of the physical phenomena.
In one embodiment, the voltage level (bias) of either the sensing or reference electrode can be switched relative to the other upon receipt of a triggering pulse. This changes the polarity of the electric field to cause charge of the opposite polarity to be driven to the sensing electrode, thereby eliminating the need to electrically connect a discharge path to the sensing electrode to clear the charge accumulated at the sensing electrode. This can be supplemented by capacitively coupling a compensation signal to the sensing electrode to cause the amplifier output signal to lessen in magnitude below a threshold level that permits additional charge or current measurements of the same polarity before performing bias reversal. These methods of clearing accumulated charge are a significant improvement over standard circuits which employ electrically noisy and expensive high value resistors to remove the charge accumulated on the sensing electrode, since elimination of this noise improves the performance of sensors which produce small currents by increasing their lower limits of detection, dynamic range and/or linearity.
Alternately or in combination with bias reversal and capacitive compensation, sensor performance can be improved by minimizing inaccuracies caused by leakage currents or current drawn from the sensor. The leakage currents are neutralized by the use of a controlled current source that induces current of a polarity opposite to the leakage current to flow to the sensing electrode. The controlled current source can be replaced or supplemented by a rate compensation circuit that compares the sensor output to the rate measured by an amplifier not connected to the sensing electrode, thereby providing an indication of the effect that the leakage current has on the sensor output to allow its correction. Other described methods of reducing leakage currents that can be used alone or in combination with the aforementioned features include the use of guard rings, physical switches or relays, the controlled creation of charges or currents of a specific polarity in a specific region of the sensing medium, controlled leakage over the surface of an insulator, and controlling the environment in which the circuit operates.
One preferred embodiment of the invention is described for use in an ionizing radiation detector which accurately measures both the total radiation exposure (with total charge reflecting the total dose) and the rate of exposure (with current reflecting the dose rate) using ionizable gas as the sensing medium. Alternate preferred embodiments of the invention permit use of the invention with sensing media other than ion chambers, including piezoelectric materials, photoelectric materials, liquid electrochemical materials, vacuum/gas/liquid/solid charge conduction materials, inductive pick-ups or coils, electric field measuring antennas, or even a surface with charge emission or work function changes. The electrical properties and/or size of many of these alternate sensing media do not require physically coupling the signal processing circuitry to the sensing device using an integrated circuit, thereby allowing use of discrete components to process the detected signal. However, where small size or low power is desired, and the number of devices to be produced is sufficient to justify the design and manufacturing costs, the circuit can be implemented on an integrated circuit. Approaches are described for manufacturing the invention to eliminate sources of leakage currents, including opening protective conductors after manufacture as needed.
Another preferred embodiment of the invention provides for a wide dynamic range amplifier which uses controlled feedback based upon or derived from the output signal. This feedback allows the measurement of small signal changes over the full dynamic range of the amplifier to a precision similar to that for the smallest signals detectable. In selected embodiments, the feedback is digitally controlled. The digitally controlled feedback allows significantly more operational or algorithmic flexibility than the analog feedback normally used in amplifiers.
Another preferred embodiment of the invention provides an improved sensing array which minimizes or eliminates all the above mentioned sources of image noise by providing a direct digital accumulation and transmission of charge or signal information rather than performing an analog accumulation of charge per pixel and a subsequent digital readout and processing thereafter. A number of circuits to accomplish this are disclosed herein. An embodiment operates by collecting charge until the threshold of a comparator is reached, incrementing a digital counter when the comparator is triggered to register a measurement, and then either removing or compensating for the collected charge so that the signal from the comparator is returned to its untriggered state to allow it to register another measurement.
In many of these embodiments the number in the digital counter represents the integral of the signal which has occurred since the counter had last been reset. Blooming is eliminated because charge never accumulates above a low threshold. Sensitivity is increased by decreasing the pixel input capacitance. The saturation level is determined by the number of bits in the counter, not the input capacitance. To double the saturation limit, rather than doubling the area of the pixel, another bit is simply added to the counter. All transport of array or pixel image information is thereafter digital information, either serial or in parallel, with its inherent noise immunity. The digital output is also amenable to simplified improvement/compensation via various existing or yet to be developed correction and compensation strategies.
By converting the signal from each pixel to a digital number during the signal collection time, blooming and saturation can be eliminated or reduced. In addition, it reduces the noise added by the circuit because an ADC that operates over a shorter time, for instance during the readout, has a greater noise bandwidth effect than the conversion strategy utilized here.
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings.